A basic ferroelectric non-volatile memory cell includes, as the major memory component, a capacitor with a dielectric which is ferroelectric material. By definition, the ferroelectric material has a permanent polarization, i.e., a non-zero internal polarization with no applied electric field. Hereinafter the term zero electric field includes no applied electric field, a zero applied electric field, or any other situation in which the resulting external electric field on the ferroelectric material is zero.
In some prior art ferroelectric memory cells pairs of ferroelectric capacitors and pairs of switching transistors are utilized. In these devices the pairs of capacitors are normally in complimentary polarization states and, thus, the states are determined by comparing the state of one of the capacitors of the pair of capacitors to the state of the other capacitor. This memory cell has the disadvantage of requiring a substantially larger number of components, which increases the size and cost of the structure.
A major problem in sensing the polarization state of the ferroelectric capacitor in a single capacitor memory cell is the fact that the electric field/polarization characteristic loop (hysterisis curve) of a ferroelectric capacitor changes over time, due to ageing from use or due to ageing from being left in a polarization state for an extended time. Generally, this change in polarization properties with time, results in a collapsing of the hysterisis curve. This is a basic materials phenomenon which is due to a non-reversibility in at least a portion of the volume of the ferroelectric material under electric field/polarization cycling. This changing of the ferroelectric material makes it very difficult to use a conventional reference cell strategy to determine the polarization state of ferroelectric memory cells.